# How Almon Strowger Invented the Automatic Telephone Switch

> An 1891 patent for an automatic telephone exchange that allowed callers to connect to each other without needing a human operator.

- **Patent:** US 447918
- **Original title:** Automatic telephone-exchange
- **Owner:** Almon B. Strowger
- **Granted:** 1891
- **Status:** Active
- **Times cited:** 70
- **Field:** telecommunications, mechanical

## What it does

This patent describes a mechanical switching system that replaced manual telephone operators. It uses a series of electrical impulses sent from a user's telephone to move a contact arm across a grid of terminals. By counting the pulses, the system physically rotates and lifts the arm to land on the specific wire connected to the desired recipient. This allowed a caller to establish a direct connection to another subscriber through a series of electromagnetic relays.

## What it does NOT cover

- Does not cover electronic or digital switching systems used in modern networks
- Does not cover software-based call routing or VoIP technology
- Does not cover systems that do not rely on physical, mechanical movement of contact arms

## The clever bit

The system used the caller's own actions to drive the switching logic, effectively turning the telephone dial into a remote control for the central office equipment.

## Real-world examples

1. The Strowger switch
2. Step-by-step telephone exchanges
3. Legacy electromechanical public switched telephone networks

## Why it matters

Before this invention, every telephone call required a human operator to physically plug a cord into a switchboard. Strowger's invention enabled the growth of the telephone network by making it scalable and private. It laid the foundation for the entire concept of automated telecommunications infrastructure.

## Frequently asked questions

### What does How Almon Strowger Invented the Automatic Telephone Switch cover?

An 1891 patent for an automatic telephone exchange that allowed callers to connect to each other without needing a human operator.

### Who owns patent US 447918?

Almon B. Strowger owns this patent, granted in 1891.

### When does this patent expire?

This patent has expired and is now in the public domain — anyone can use the invention freely.

### What is patent US 447918 cited by?

This patent has been cited by 70 later patents that build on its ideas.

### What problem does this patent solve?

Before this invention, every telephone call required a human operator to physically plug a cord into a switchboard. Strowger's invention enabled the growth of the telephone network by making it scalable and private. It laid the foundation for the entire concept of automated telecommunications infrastructure.

### What does this patent NOT cover?

Does not cover electronic or digital switching systems used in modern networks

**Full plain-English explainer:** https://patentbrief.org/patent/us/447918/strowger-automatic-telephone-exchange

**Original patent:** https://patents.google.com/patent/US447918

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_Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._


## Related patents

Semantically similar inventions in the PatentBrief corpus:

- [How Early Cell Phones Handled Calls Across Different Towers](https://patentbrief.org/patent/us/3906166/cellular-mobile-phone-radio-telephone) — This patent describes a system for early portable phones to automatically find the strongest signal from a base station and switch channels as the user moves, reducing battery drain and interference.
- [Alexander Graham Bell's Patent for the Telephone](https://patentbrief.org/patent/us/174465/bell-telephone) — Alexander Graham Bell's 1876 patent describing the method and apparatus for transmitting vocal sounds telegraphically, effectively inventing the telephone.
- [How Samuel Morse Patented the Electric Telegraph System](https://patentbrief.org/patent/us/1647/morse-telegraph) — Samuel Morse's 1840 patent for the electric telegraph, which enabled long-distance communication by sending electrical pulses over wires to represent letters.
- [How the First Automatic Pop-Up Toaster Works](https://patentbrief.org/patent/us/1394450/pop-up-toaster-strite) — Charles Strite's 1921 patent for the first toaster that automatically pops bread up after a set time, preventing it from burning.
- [How the QWERTY Keyboard Layout Was Originally Designed](https://patentbrief.org/patent/us/207559/qwerty-typewriter-sholes) — An 1878 patent by Christopher Latham Sholes that helped standardize the keyboard layout we still use on computers and phones today.